Polymerizable acrylic acids undesirably polymerize during various stages of the manufacturing, processing, handling, storage and use thereof. One especially troublesome problem is the polymerization of acrylic acid monomer in the purification stages of monomer production. It is well known that the monomer readily polymerize and that such polymerization increases with concurrent temperature increases.
Polymers are generally formed by free radical chain reactions. These reactions, shown below, consist of two phases, an initiation phase and a propagation phase. In Reaction 1, the chain initiation reaction, a free radical represented by R.cndot., is formed (the symbol R.cndot. can be any hydrocarbon). These free radicals, which have an odd electron, act as chain carriers. During chain propagation, additional free radicals are formed and the hydrocarbon molecules (R) grow larger and larger (see Reaction 2c), forming the unwanted polymers which accumulate on heat transfer surfaces.
Chain reactions can be triggered in several ways. In Reaction 1, heat starts the chain. Example: When a reactive molecule such as an olefin or a diolefin is heated, a free radical is produced. Another way a chain reaction starts is shown in Reaction 3. Metal ions initiate free radical formation here. Accelerating polymerization by oxygen and metals can be seen by reviewing Reactions 2 and 3.
As polymers form, more polymers begin to adhere to the heat transfer surfaces. This adherence results in dehydrogenation of the hydrocarbon and eventually the polymer is converted to coke.
1. Chain Initiation EQU R--H.fwdarw.R.cndot.+H.cndot.
2. Chain Propagation
a. R.cndot.+O.sub.2 .fwdarw.R--O--O.cndot. PA1 b. R--O--O.cndot.+R'--H.fwdarw.R'.cndot.+R--O--O--H PA1 c. R'.cndot.+C=C.fwdarw.R'--C--C.cndot..fwdarw.Polymer PA1 a. Me.sup.++ +RH.fwdarw.Me.sup.+ +R.cndot.+H.sup.+ PA1 b. Me.sup.++ +R--O--O--H.fwdarw.Me.sup.+ +R--O--O.cndot.+H.sup.+ PA1 a. R.cndot.+R.cndot..fwdarw.R--R' PA1 b. R.cndot.+R--O--O.cndot..fwdarw.R--O--O--R
3. Chain Initiation
4. Chain Termination
Research indicates that even very small amounts of oxygen can cause or accelerate polymerization. Accordingly, to inhibit this insidious fouling problem, it is highly desirable to provide a polyfunctional process antifoulant which can, among other functions, inhibit oxygen based polymerization initiation. This antioxidant function serves as a "chain-stopper" by forming inert molecules with the free radicals similar to the chain termination as indiciated in reactions 4a and 4b.
Common industrial methods for producing acrylic acids include a variety of purification processes, including distillation to remove impurities. Unfortunately, purification operations carried out at elevated temperatures result in an increased rate of undesired polymerization. Polymerization, such as thermal polymerization, during the monomer purification process, results not only in loss of desired monomer end-product, but also in loss of production efficiency caused by polymer formation or agglomeration on process equipment. In heat requiring operations, such agglomeration adversely affects heat transfer efficiency.